anything is 'plausible' if you set it up to make sense in the context of your plot and can write it well enough to put it across...

the reason for such an acceptance of death [and/or violence] by an entire populace, as nothing to get worked up over, could be a religious belief, or merely that death and violence are so common there, they ceased to be anything worth noting... or it could be that the population is controlled by drugs in the water supply or whatever, so that emotions are dulled and the girl has somehow not been affected, so her normal emotions come into play... btw, that last one was used very successfully in heinlein's 'the puppetmasters' and other sci-fi works...

In Brave New World, Huxley wrote it so children constantly visited the dying and were conditioned to accept it as an inevitable occurence not worth considering.

But if the people are not too distracted (with soma, in Brave New World), then death becomes something much, much more. Religion can placate it some, but it needs to be in a way that would nearly advocate death, as opposed to many current religions preaching no harm.

1984 had a version of this called "Death Worship". Obviously we can assume since this was an INGSOC word, "Death Worship" was the Orwellian form of Buddhism, which preaches rebirth. A society conditioned to death would have to unkillable faith in whatever cause they're fighting for, simple as that.

When you feel pain, sometimes your nervous system do this weird thing where they make your body desensitive to pain. They just shut down and you don't feel pain anymore- that will most likely happen in a life threatening situation, though.

I would say Islamic Terrorist are desensitized to death for the most part. If you are able to blow your self up, you don't have much fear of death. The Japanese samurais also seemed to have no fear of death. I do not have a fear of death. I used to when I was younger. The only thing uncomfortable about death for me is how I die.

Religious conditioning, or mental conditioning in general could desensitize a people to violence. Perhaps they have laws that children are handed over for public training at age 4. Just like we are conditioned in public school, they condition them to not fear violence or be emotionally effected by it. A rigorous training program for every human being.